In response to PA-12-279, the goal of this R34 is to refine and examine the feasibility, acceptability and preliminary impact of a theoretically guided intervention that provides system 'boundary-spanning' services to improve mental health service use and outcomes for low-income, transition-age youth with mental disorders (TAYMD). Through the refinement and testing of this intervention, called Cornerstone, we aim to address mental health challenges as well as the practical obstacles, such as lack of education, housing, and employment, that impede successful transition to adulthood. Cornerstone addresses these challenges by providing a service delivery strategy that spans the transition from the child to the adult system. Cornerstone centers on addressing three pressing problems: (1) the discontinuation, or at best fragmentation, of mental health care for TAYMD, (2) the lack of evidence-based interventions for TAYMD, and (3) the reality that promising practices for TAYMD, many of which Cornerstone builds upon, have not been tested with a sample that moves from child to adult services. Cornerstone deals with these problems in innovative ways at a time of transformation in health care. First, Cornerstone provides a 'boundary spanning case manager' (BSCM) across the transition. That is, rather than extending the age of service provision in the child system, we prioritize collaboration with the adult system and practical assistance to assure that the transition to developmentally and clinically appropriate adult services and independent adulthood happen at age 18. Second, Cornerstone incorporates an innovative form of social support, namely a recovery role model mentor (RRM) who is a decade older than TAYMD. The RRM co-facilitates weekly groups and provides important information and mentoring (i.e., modeling, connection) for TAYMD. Third, the development of Cornerstone is aligned with policy and practice transformation, particularly Affordable Care Act and New York State Medicaid Redesign, prioritizing coordinated, evidence-informed care for TAYMD, expansion of the workforce, and an emphasis on achieving functional outcomes and wellness. The aims are 1) to develop and refine all manuals and protocols for Cornerstone, 2) to determine the feasibility, acceptability, and preliminary impact of 'Cornerstone' relative to treatment as usual (TAU) on mediating outcomes (e.g., stigma), service use, and improved mental health and functioning outcomes, and 3) to explore implementation of Cornerstone through individual and group interviews with key stakeholder partners on aspects of the transforming local, state and national service context (e.g., staffing, payment). As a multidisciplinary team of professionals on the local, state, and national level we are committed to bridging the science to service gap for youth transitioning to adulthood with mental health challenges. Using qualitative methods and an RCT, this study is the first to examine a true transition intervention. We believe mental health services research that focuses on the transition to adulthood with innovative service delivery strategies that span developmental silos will decrease the number of TAYMD with unmet mental health needs.